If you look at the sacrifices that they made in separate lights, without realizing what the other person has sacrificed, they appear to be pretty sensible. After all, what does Della really need long hair for? It's just hair, and she tells Jim later on, "It'll grow back." Hair is just hair, and if she can get some money for it, in order to try to show her husband, who has been working so hard, how much she loves him, then that doesn't sound too kooky or illogical to me. It's rather sweet.
Then, take Jim's sacrifice. He gives up his watch. It's just a watch--granted, he loved it, but, things are just things, and if giving it up can show Della how much he loves her, that makes sense to me. He can get a cheaper watch, or use clocks in the room to tell time--it's not like he will die without a watch, just like Della won't die without her hair.
It's only when you combine the two presents that these two people get that the combination becomes insensible; Jim gets Della combs for hair that she no longer has, and Della gets Jim a chain for a watch that he no longer has. It is an unfortunate comination of sacrifices and presents, and it is that combination that makes their original sacrifice look insensible. But, in reality, taking them separately, they were nice gifts to give.
One other thing to consider is the difficulty of using the word "sensible" to describe love. How "sensible" is love, really? How "sensible" are gifts that we give to our loved ones? The point of giving gifts to show our love is not to be "sensible," but to show them that we care, that we notice what makes them happy. Giving "sensible" gifts is boring, and not very romantic. So trying to describe their sacrifice as sensible is really missing the point entirely.
I hope that those thoughts help get you started; good luck!
Exchange of gifts testifies to exchange of love, love which is true and mutual. The young couple in The Gift of the Magi, Jim and Della, made such an exchange to show their tue love and adoration for each other.
Della nurtured her beautiful asset of long, brown, cascade-like hair which she sold to raise some cash so that she could buy an appropriate Christmas gift-a platinum chain exactly suited to Jim's gold watch--for her husband. On the other hand, Jim chose to sell his very fond & proud possession, his watch, to procure the money to buy a set of wonderful combs perfectly matched to Della's lock of hair.
What Della and Jim did may be looked upon as insensible, foolish acts if those acts are viewed in isolation and from an individual's personal point of view. But since they loved each other and genuinely thought of expressing their love in the form of Cristmas gifts, they must have acted quite sensibly. Love is more valuable than a lock of hair or a watch, and needs validation through selfless sacrifice; Jim and Della were the true magi.